This invention relates to an apparatus for the preparation of a reaction mixture, in particular one which forms a foam, from fluid components, consisting of storage tanks from which feed pipes lead through metering pumps to a mixing head and open into a mixing chamber through injection apertures. The mixing chamber is formed by the bore of a housing in which the control piston of a control device is displaceable, and return lines are provided in the system.
Apparatus of this type may be used for the preparation of reaction mixtures which give rise to cellular materials such as flexible, semi-rigid, rigid or structural foams (e.g. based on polyurethane or isocyanurate) or non-cellular materials such as elastomers.
The production of shaped products by the reaction of a mixture in a mold is becoming increasingly important, particularly in the automotive industry for the production of seat shells, dashboards, steering wheels, crash pads, bumpers and soft faces. Because of the volumes involved, it is essential to ensure satisfactory and troublefree operation.
The flow of the reactants from "circulation" to "mixing" and conversely during continuous operation entails considerable difficulties. Exactly synchronized entry of the various reactants, which may differ in their viscosities and quantitative rates of throughput, is of the greatest importance for obtaining perfect shaped products. Even the slightest racing ahead of one component produces faults in the finished product. Furthermore, the volumetric capacity of the mixing chamber must be adapted to the total quantitative throughput. In other words, relatively small rates of throughput (e.g. of less than 5 kg/min) require the use of very narrow mixing chambers. At the present time, there is no completely satisfactory apparatus on the market which will fulfill these requirements.
Some mixing heads are equipped with hydraulically operated needle circulation nozzles. Synchronized flow from circulation to mixing is effected by a hydraulic servo system. This entails certain sources of error such as air in the hydraulic system obstruction of a nozzle, and the like. Exact positive control is thus extremely difficult, if not impossible. The use of such nozzles also requires an additional driven ram for cleaning the mixing chamber. The whole construction is therefore quite complicated. Characteristic of these types of mixing heads are those described in U.S. Pat. Nos. 3,926,219 and 4,105,045.
Other mixing heads are known in which the flow from circulation to mixing of the components is effected by means of circulation grooves formed in the control pistons of a control device. Characteristic of these types of mixing heads are those described in U.S. Pat. Nos. 3,706,515, 3,975,128, 3,982,870, 4,129,636, and 4,013,391. These also have disadvantages. The pressure of the components is adjusted by nozzles. The pressure during circulation should be essentially the same as during mixing but this is difficult to achieve since the pressure losses in the circulation grooves and the pipes following are added to the pressures produced by the injection nozzles. Moreover, it is not possible to use small control piston diameters with these mix heads since the two circulation grooves must be separated from each other. The volume of the mixing chamber is therefore very restricted, especially for relatively small rates of throughput. When small diameters are used, the problem of providing an effective seal between the circulation grooves of the control piston for the various reactants becomes extremely difficult. Moreover, in the case of small circulation grooves there is a disproportionate increase in pressure when the system is switched to "circulation".
It is, therefore, an object of the present invention to produce an apparatus which provides exact positive control of the components, a simple method of control, great variability in the adjustment and construction of the volume of the mixing chamber (particularly for relatively small rates of output), and a simple and, if desired, independent and separate adaptation of the circulation pressure to the injection pressure.